The automatic volume control (AVC) of this invention is a fully automatic system and method for adjusting the volume of an audio output device, such as a car radio, in accordance with listener preferences, to compensate selectively for changing levels of ambient noise only in the time and frequency domains that interfere with intelligibility of speech or appreciation of music.
An example of an audio device is a car radio. Many sources of noise can interfere with hearing a car radio, including tire (road) noise, wind, engine noise, traffic (highway) noise, the fan of a heater or air conditioner, and noises made by the driver and passengers. The noise levels of all of these sources can change with time, depending on factors like the speed of the car or changing environmental conditions outside or inside the car. The noise levels can change abruptly or quasi-continuously or can be transient. Having repeatedly to manually adjust the volume of an audio device to compensate for changing noise levels is a nuisance, and, in a car, can compromise the safety of the occupants and others.
Not all noise, however, interferes with a listener's understanding or appreciation of the output of an audio device. And not all noise, therefore, would impel a listener to want to change the volume. For example, nearly all the information in speech is contained within the frequency interval 200 Hz to 6 kHz [L. E. Kinsler et al., Fundamentals of Acoustics, Third Ed. (John Wiley & Sons, NY, 1982), p. 283]. Generally, only the frequency components of noise within this interval can detract significantly from intelligibility of speech. Similarly, the intelligibility of full sentences in noise environments is substantially greater than the intelligibility of isolated words. Generally, only noises that persist long enough to mask more than a few words can detract significantly from intelligibility of speech.
Any system that attempts to compensate for all noise, regardless of frequency or duration, will generally overcompensate by raising or lowering the volume of an audio device to adjust for noise that is not significantly interfering with the ability to listen to the audio device. For example, the occurrence of a high-pitched whine above 6 kHz should not generally be cause for the volume of an audio device to be increased automatically, or to be decreased upon its cessation. Similarly, a transient shout within a car, or another car passing at high speed in the opposite direction, should not generally be cause for the volume to be changed.
What is needed, therefore, is not a means for automatically adjusting the volume of an audio device to compensate for changes in all ambient noise, but rather only that noise of a frequency and duration that detracts from the ability to listen to the audio device. That is, the AVC should have some means of discriminating significant noise, which persistently detracts from listening ability, from noise that is less consequential. One means of identifying such significant noise is to measure its interference with the intelligibility of speech. One measure of interference with intelligibility considered suitable for field use is the preferred speech interference level (PSIL), which is the arithmetic average of the noise levels in the three octave bands centered at 500, 1000, and 2000 Hz [ibid., p. 284].
To be fully automatic, an AVC should impose no need for additional manual controls on an audio device, other than possibly an on-off switch for the AVC feature. Listener preferences for volume should be established through normal operation of the audio device and a minimum of manual volume adjustments. The two key listener preferences that should be automatically registered by an AVC are the preferred signal-to-noise ratio and the preferred signal floor. The relevant signal-to-noise ratio is the ratio of the amplifier gain of an audio device to a suitable measure of significant noise, such as the PSIL. The preferred signal floor is the lowest amplifier gain acceptable to the listener, independent of how quiet the environment may be.